1. Field of the Invention
The present invention generally relates to a power amplifier, in particular, to a power amplifier which dynamically adjusts the dead time between signals therein for controlling a switching circuit so as to reduce the common noise thereof.
2. Description of Related Art
Power amplifier plays a very important role in integrated circuit (IC) design, and which is broadly applied to radio communication equipments, transmitters and receivers in television broadcasting, high-fidelity stereo equipments, micro computers, and other electronic equipments. A power amplifier is used for increasing the power of a signal so that the signal can be used for driving a next-level circuit. Accordingly, the performance of a power amplifier is determined by the power gain thereof, wherein the power gain of the power amplifier refers to a ratio of the output power of the power amplifier to the input power of the power amplifier.
The higher the power gain of a power amplifier is, the better performance the power amplifier has. Usually, the power gain curve of a power amplifier has good linearity when the input signal is weak. However, when the input signal is very strong, the power amplifier cannot amplify the input signal linearly, and accordingly the power gain of the power amplifier is reduced. Foregoing phenomenon is referred as gain compression. Along with the increase of an input signal, the later the gain compression of the power amplifier takes place, the higher linearity the power amplifier has. A power amplifier of high linearity has good performance in outputting undistorted signals.
Power amplifiers can be categorized into many different classes, such as class A, class B, class AB, class C, and class D, according to their applications. For example, power amplifiers of class D are broadly applied to audio signal processing in handheld or mobile apparatuses due to the high power conversion efficiency (>90%) thereof. Besides, a pulse width modulator (PWM) may be adopted by a D-class power amplifier for producing continuous pulses, and the pulse width changes along the range of the audio signals, so as to control the operation of a switching circuit in the D-class power amplifier. However, the performance of a D-class power amplifier is not as good as that of an AB-class power amplifier when applied to a product having high demand to signal distortion.
Accordingly, to improve the linearity of an output signal of D-class power amplifier, a sigma-delta D-class power amplifier has been provided. The sigma-delta D-class power amplifier has lower signal distortion compared to AB-class power amplifier and still keeps the high power conversion efficiency of D-class power amplifier. Thus, the sigma-delta D-class power amplifier has high competitiveness in the power amplifier market. However, the sigma-delta D-class power amplifier has a fatal disadvantage, which is, when the input signal is increased to a specific extent (usually, half of a reference level), the total harmonic distortion plus noise (THD+N) of the sigma-delta D-class power amplifier will increase drastically, wherein THD+N is a ratio of the total of harmonic distortion and noise produced by an equipment to the output power of the equipment.
A “sigma delta modulator with reducing switching rate for use in class D amplification” has been disclosed in U.S. Pat. No. 6,924,757. According to this disclosure, the swing of an input signal is detected by an input signal swing detector, and the hysteretic range of a quantizer is determined through table lookup. When an input signal of the sigma delta modulator is increased, the hysteretic range of the quantizer is also increased, and when the input signal is reduced, the hysteretic range of the quantizer is also reduced, so that both the stability and the signal-to-noise ratio (SNR) of the signal are improved.
Since the quantizer has a hysteretic range, the average clock of the sigma delta modulator is reduced, so that the power loss of a switching circuit in a D-class power amplifier (output stage) is reduced and accordingly the performance of the power amplifier in THD+N is improved. However, in the disclosure described above, the swing of the input signal has to be detected first, and then an appropriate hysteretic range is selected through table lookup and converted into a hysteretic control signal by a conversion circuit. Thereby, the complexity, power consumption, and fabrication cost of the system circuit are all increased considerably.